There are a number of prior art patents related to synthesis of aluminum phosphate materials primarily for use as a catalyst support including crystalline and amorphous forms. Most synthetic methods comprise of using a sol-gel technique with raw materials that include commonly available salts of aluminum and a variety of phosphorous sources including phosphoric acid, ammonium hydrogen phosphates, phosphorous acid, and others. Many of these methods yield highly porous and crystalline forms and few thermally stable amorphous compositions (U.S. Pat. No. 4,289,863, Hill et al.; U.S. Pat. Nos. 5,698,758 and 5,552,361, both Rieser et al.; U.S. Pat. No. 6,022,513, Pecoraro et al. U.S. Pat. No. 3,943,231, Wasel-Nielen et al.; U.S. Pat. No. 5,030,431, Glemza; U.S. Pat. No. 5,292,701, Glemza et al.; U.S. Pat. No. 5,496,529 and U.S. Pat. No. 5,707,442, both Fogel et al.). Two prior art patents do teach formation of amorphous aluminum phosphate compositions. However, the materials derived are highly porous which are desired for catalytic applications. U.S. Pat. No. 4,289,863, teaches a new method for synthesizing amorphous Al-rich AlPO4 compositions which are more thermally stable than Al-poor compositions which crystallize at much lower temperatures. U.S. Pat. No. 6,022,513, teaches a slightly modified method for making Al-rich compositions which yields a microstructurally different form of amorphous aluminophosphate material. However, both synthetic methods yield highly porous materials with surface areas over 90 to 300 square meters per gram with a macropore volume of at least 0.1 ccs per gram as shown in the Pecoraro patent. (pores are said to be between 60 nm to 1000 nm in U.S. Pat. No. 5,698,758).
Much of the utility of such prior art amorphous materials is related to their use as thin films on metals & alloys, glass, and ceramic substrates. To facilitate this utility, a combination of additional attributes would be advantageous including a stable and low-cost precursor solution and an environmentally-friendly, cost-effective, and versatile coating process providing good adhesion with aforementioned substrates. There is a growing need for coatings on glass and ceramic substrates to provide protection and to perform other surface-related functions. The primary advantage of an amorphous coating is that, if developed by a suitable process, it can provide a hermetic seal over a substrate such that access of gas or liquids that can potentially corrode the substrate is avoided. Many methods have been developed to deposit uniform crystalline coatings that are substantially pore or crack-free. Crystalline coatings do not provide hermetic protection from gas or liquid exposures.
Silica-based amorphous coatings have been developed and a recent patent prescribes a unique way to deposit such coatings (U.S. Pat. No. 6,162,498). However, the coating is not durable under certain harsh conditions and are not thermally stable at elevated temperatures or do not serve adequately as a transparent coating on glass due to processing limitations. In addition, silica is not chemically compatible with many ceramic materials at elevated temperatures and is also prone to decomposition in water vapor atmospheres (converted to hydroxide vapors). High temperature stable glassy or vitreous coatings have also been developed by initially coating substrates with a slurry of glass frits and subsequently treating the coated material to high enough temperatures to melt the glass frits and form the vitreous coating. Vitreous enamel coatings have been in existence for many decades with many different compositions. However, they are usually thick and are porous and deform at elevated temperatures. Although hermetic protection may be achieved with this process, the requirement of high temperature processing to melt the glass frits may degrade the substrate and if low melting glass compositions are selected, they may not be durable due to the presence of sodium.
Prior art coatings have also included amorphous aluminum phosphate on metals derived from various methods. British Pat. Nos. 1,322,722, 1,322,724, and 1,322,726, and published article entitled “Novel, low curing temperature, glassy, inorganic coatings, derived from soluble complexes of aluminum and other metal phosphates”, (Chemistry and Industry, vol. 1, (1974) 457-459) disclose utilizing a soluble polymer complex comprising of aluminum phosphate with HCl and hydroxyl-organic ligand. Although dense amorphous aluminum phosphate films have been reported utilizing this method, there are several shortcomings which relate to their poor performance and make it impractical for commercial use. First, the films contain residual chlorine (minimum of one weight %) which is not desirable for many metals and alloys. Second, as the film cures, toxic HCl gas is released (complex contains one mole HCl for every mole of AlPO4) which is a significant environmental concern. Third, the synthetic process is relatively complex involving isolation of the complex in crystalline form and then dissolving it in appropriate solvents making it difficult to implement in practical applications.
Inert and/or vacuum treatments are necessary to produce the precursor in the aforementioned prior art and, in addition, it is not clear whether the prepared precursor solution has sufficient shelf stability, or if the solution decomposes upon exposure to the ambient (a potential concern due to the presence of volatile organics, such as ethanol, present as a ligand). No specific examples were given related to deposition of films on metal substrates or their corresponding behavior in an oxidation or corrosion tests. Due to the highly acidic nature of the precursor solution, glass and ceramic substrates may be subjected to significant corrosion from chloride attack during film development. In addition, due to the lower curing temperature, adhesion to substrates may not be sufficiently high to yield durable films. Although curing temperatures ranging from 200-500° C. were suggested, most often curing temperatures below 200° C. were used and no specific example of films cured at 500° C. was provided and no microstructural information was given. In addition, the coatings were found to adhere to molten aluminum. However, Aluminum phosphate, in pure crystalline or amorphous forms, is chemically compatible with molten aluminum and has been found to be non-wetting due to low surface energy. Based on the poor adhesion of the prior art coatings, it is suspected that the coating is not chemically durable (due to presence of chlorine or poor film coverage or poor high temperature properties) and that the surface energy is not sufficiently low such that its applicability for non-stick or non-wetting applications may not be exploited.
In the aforementioned prior art, in addition, silicon and boron additions were needed to extend the amorphous nature of the material. Even with these additions, sufficient crystalline content (tridymite and cristobalite) was present after annealing the powder materials to 1090° C. for 3 hours. As explained below, for the present invention, substantial amounts of non-crystalline content with only the presence of tridymite phase were found for materials with varying Al/P stoichiometry after heat treatment at much higher temperatures and extended time periods. It is not uncommon that amorphous materials produced using various techniques may have distinct structural or network moieties such that their atom diffusivities and high temperature behavior may vary significantly. It appears that the network structure of the material derived under the aforementioned patent does not provide for a robust microstructure and may not be suitable for use especially at elevated temperatures.
Thus, the material produced in prior-art methods is not microstructurally dense or robust enough to provide the desired protection. In addition, none of the prior art methods provide a suitable process or precursor solution that is economical, stable and clear, and can be applied using a variety of well-known techniques such as dip, spray, brush, and flow. Furthermore, none of the processes associated with prior art methods offer the ability to provide good adhesion with substrates that is critically important for most applications. The prior art coatings are either not durable under certain atmospheric conditions or under certain harsh industrial or use environments where materials are subjected to thermal treatments or exposed to corrosive environments. Prior art inorganic coatings are also not completely transparent for use on glass where transmission properties are affected or other substrates where aesthetic property of the substrate (metallic appearance) needs to be preserved.
The technical demands on the glass industry are growing for display technologies, energy-efficient windows, efficient solar panels, mirrors and lenses, and other specialty products. It is anticipated that suitable coatings with multifunctional properties (for example, good diffusion barrier characteristics and provide antireflective properties) will be needed to meet the demands of performance, durability, and cost. The currently-available commercial coatings do not adequately provide the combination of desirable properties, in addition to, not providing simple and low-cost processes for deposition of multifunctional thin films.
The container market, for example, use coatings to strengthen the glass and prevent it from shattering during the manufacturing and handling. The flat-glass market has numerous products, whose performance and lifetime directly depends on the quality of the coating. Low-E coatings (which transmit visible light while minimizing the transmittance of other wavelengths of light, such as light in the infrared spectrum) or electrochromic coatings (also called “smart windows”), whose improved transmission properties will yield energy savings. Another application has raised the interest of the flat-glass manufacturers: easy-to-clean or self-cleaning windows for homes and buildings, lenses and mirrors for the optical industry. Self-cleaning or easy-to-clean coatings are designed to improve visibility, lower labor costs, minimize detergent use and to allow efficient performance of underlying optical coatings. The extant coating technologies, whether hydrophilic or hydrophobic (In the case of hydrophobic coatings, organic or polymer coatings form a water-repellent surface. Hydrophilic coatings combined the action of a photocatalyst, mostly TiO2, with hydrophilic properties of the surface to clean away the loosen dirt with water) suffer from severe limitations in either process or performance.
Although several prior art patents (U.S. Pat. No. 6,379,746, PCT Application: WO 2001-US42881, PCT Application: WO 98-US21797) relate to the development of coatings for specifically addressing various issues aforementioned, no suitable multifunctional coating material is presently available that meets more than one requirement and can be deposited using a low-cost and simple deposition process. In addition, most glass articles comprise of soda-lime glass which contain sodium ions that diffuse into coating layers being deposited leading to deterioration of functional properties. For example, transparent conductive oxides (such as indium tin oxide) are used in a variety of glass products to limit IR radiation or to serve as conductive layers in display devices or solar cells do not perform adequately due to the diffusion of sodium from the substrate. Thus, a robust diffusion barrier layer on glass is needed to fully exploit the functionality of overlayers being deposited.
Ceramic articles are used in a number of industries including tiles, porcelain, refractories, bricks, furnace liners, and other specialty products. Such ceramic articles produced by any number of processes are porous and rough, thus requiring a glaze coating that provide resistance against staining, scratching, UV radiation, and fire. In addition, they are not easy to clean as dirt or foreign particles absorb into the pores rather easily. Glazing of such ceramic surfaces to provide a seal coat are used extensively. However, the performance of such glazes are rather poor and are typically thick coatings which alter the surface morphology significantly such that anti-slip properties are compromised. In addition, certain glazes are polymer-based and are not durable.
Porcelain enamel coatings are used extensively in the ceramic industry (including, but not limited to, BBQ & stove grills, household appliances, chemical, heat treating, metal, and molten metal processing industries). Bulk of these coatings are porous which result in their poor performance (moisture absorption, dirt collection, etc.). In addition, the surfaces are not suitably non-wetting or non-stick or hydrophobic.
In addition, many ceramic materials are used in high temperature applications. Non-oxide ceramic materials, such as silicon nitride, silicon carbide, and their related composite materials are known for their excellent mechanical (creep resistance and strength at elevated temperatures) and thermal shock properties. However, these materials are subject to environmental degradation and rapid oxidation in harsh and oxidizing environments. A suitable coating that provides oxidation or corrosion protection is highly desirable. Currently-used protective coatings are thick and are prone to cracking. A thin, thermally stable, and microstructurally dense amorphous film with low oxygen diffusivity or atom mobility can provide excellent protection. In addition, suitable dielectric layers are being sought for silicon carbide based semiconductors for high temperature applications. Silica is commonly used as a dielectric layer for silicon-based devices and is generated by controlled thermal oxidation of silicon. However, such films cannot be thermally grown on SiC that yield the desirable dielectric properties (low dielectric constant).
Finishing polishing of optical materials, such as lenses and radomes, is a significant challenge, particularly substrates that are hard and are polycrystalline in nature such as AlON and spinel materials used in radomes. Depositing an amorphous glassy layer with appropriate optical or electrical properties will enable ease of polishing using magnetic rheological fluids (MRF) polishing or other mechanical techniques to achieve the desired rms roughness value for the final optical surface. The glassy layer can seal defects, sub-surface damage on the substrate surface, and grain-boundary junctions.
Thus, there is a need to develop oxide coatings using low-cost, versatile, and simple processes to enable the aforementioned applications.